University of Zurich: Humans and Chimps

We made it. Our closest relatives didn’t. Over the course of evolution, humans have developed a capacity for language that is unparalleled in the animal kingdom. No other animal has a system of communication as versatile as our language. But what enabled us humans to develop our language, and why were chimpanzees , for example, unable to do the same?

The National Center of Competence in Research (NCCR) Evolving Language has brought together linguists, evolutionary biologists and anthropologists to tackle this question. The name of the NCCR points to the core of the project – the researchers want to explore the past, present and future of human language. “We want to understand why this mind-boggling form of communication came to be,” says UZH linguist Balthasar Bickel, who leads the NCCR.

Structure, sound and meaning
One of the starting points of this investigation into the origins of human language is animal languages. Many animals use simpler forms of communication, which can be seen as precursors to human language. And this applies to each of the three key features of our language: Structure, sound and meaning. Different languages assemble each of these elements in varying levels of complexity. “We want to understand which elements of our language are older and were already used by animals, and which are new and developed in humans only,” Balthasar Bickel explains. In other words, which forms of language can be found in animals, which are “exclusively” human, and when and why did they emerge?

We know that great apes such as chimpanzees or orangutans have large brains and cognitive skills that come close to matching our own. But despite this, they failed to keep up with humans. We humans succeeded in out-evolving our closest relatives, rising to the top of the animal kingdom. According to anthropologists such as UZH professor emeritus Carel van Schaik, this is due to our cumulative culture, that is, our capacity to build on the cultural accomplishments of our predecessors and pass on, expand and deepen cultural knowledge from one generation to the next. Language is an essential part of this process. It is the tool that allows us to hand down our knowledge.

Language is closely linked to the human brain, which has enabled us to develop the cognitive skills that gave rise to culture and speech. As such, it makes sense to explore the origins of language by drawing on insights from the field of evolutionary biology that explain how our powerful brains developed.

There’s food here!
One way to shed light on the origins of language is to look to the animal kingdom. Are there comparable forms of language in animals, for example in monkeys? But those aren’t the only animals that use relatively complex, human-like forms of communication – researchers have also observed this in birds and meerkats. The ability to point things out by using calls, for example. Monkeys and meerkats can both warn other members of their species when a predator is near, and they can indicate a source of food. “Chimpanzees have two distinct food-associated calls,” explains Simon Townsend, professor at the Department of Comparative Linguistics at UZH. “One call is used to indicate high-quality food such as fruit, the other for less valuable food such as leaves.” The other apes understand these calls and then know where to look for food. It also matters whom this information is shared with. “We observed that they were more likely to call when partners are near that are close to them,” says Townsend. But chimps can also suppress their calls and keep the food for themselves.

For linguists, the key takeaway here is that apes – like humans – can use their “language” purposefully and systematically. “Sharing information isn’t an exclusively human trait,” explains the language researcher.

Watch out for eagle!
Meerkats are another species with keen communication skills. Marta Manser, a behavioral biologist and UZH professor of animal behavior, regularly travels to her research station in the Kalahari Desert to investigate these squirrel-sized creatures. She also observes them in various zoos and in her own enclosure on Irchel Campus, where she can see the little predators from her desk.

Thanks to Manser’s work, the meerkat language is one of the most well-researched communication systems in all of the animal kingdom. Even though meerkats have much smaller brains than chimpanzees, the range of calls they use is about the same. Marta Manser distinguishes between different call types, including alarm calls and contact calls. Alarm calls can vary depending on the source of danger – e.g. birds of prey approaching from above, or predators on the ground such as jackals, lions or snakes – and urgency. Their behavior towards snakes is particularly remarkable. While meerkats usually escape to the nearest burrow when predators approach, when they come across a snake they gang up and try to fight it off together.

Remarkably diverse repertoire
Contact calls, in contrast, are used to let other members of the group know where they are and how they are doing. One particularly interesting example is the “sunning call”, which the animals use to signal to others that everything is okay and they’re happy where they are. Manser likens this behavior to an informal chat among friends, where the primary goal isn’t to relay urgent information that requires an immediate response (e.g. “Watch out for the eagle!”), but to reassure one another of one’s whereabouts and current state. The calls of meerkats are made up of up to 30 different sounds, says Manser. And these calls “appear to be innate,” according to the meerkat researcher. But young animals must first learn when to use which call. Their remarkably diverse repertoire of calls is based on a combination of only a few sounds, which can also be distinguished in terms of emotion and urgency. Alarms calls, for example, grow louder and more urgent the closer a predator comes. And they can indicate whether or not the enemy is moving. “The calls describe precisely what’s happening,” explains Manser, “for example: ‘The jackal is sitting! Watch out, it’s moving! Now it’s sitting again!’.”

Sounds with meaning
This combining of calls is seen as a rudimentary form of syntax, as the calls – like the words in a sentence – are joined together according to a specific set of rules and take on different meaning depending on their order. As with meerkats, the language of monkeys follows specific syntactic rules. Researchers have been able to demonstrate this in small monkey species. Some years ago, Klaus Zuberbühler, professor of comparative cognition at the University of Neuchâtel and member of the NCCR Evolving Language, showed that Campbell’s monkeys can string together sounds to form sequences (sentences) that have different meanings. Research into syntactic-like structures in great apes, though, has so far been inconclusive.

This is the current focus of Simon Townsend and his research group, and their initial findings are very promising. Together with his PhD candidate Maël Leroux, he was able to demonstrate that chimpanzees can distinguish between different combinations of alarm calls. Their research showed that the chimps use an alarm call when they encounter a threat, and they use a different call when asking for help. And when they encounter a threat and need help, they combine the two calls. “Watch out, danger! Come and help me!”

This type of combined call is used when they see a snake, for example. The researchers conducted an experiment in which they played the calls back to the chimpanzees through loudspeakers – to astonishing effect, says Townsend: “When they heard the calls, they got closer to the loudspeaker, then climbed up a tree and looked down to see if there was a snake nearby.” In other words, the animals reacted to the recorded calls in the same way as if another chimpanzee had made them then and there.

So what does this mean in terms of language evolution? “The question is whether syntax-like structures in different species are an instance of convergent evolution, that is, they developed simultaneously, or whether they evolved over time from previous forms,” explains Townsend. The fact that our closest relatives use simple forms of syntax suggests that the origins of these structures can be traced back six to seven million years. In other words, syntax in human language likely evolved from previous forms, which thanks to Townsend have now been shown to exist in chimpanzees. So far so good. But this brings us to the next question – what happened in these six million years? How come humans were able to continue developing their language, while the language of chimpanzees barely changed?

Cooperative breeding
The search for an answer brings us back to the question of how humans were able to develop the big brains needed to handle complex language. And with this, we once again find ourselves in the field of anthropology. This time, we turn to Judith Burkart, a UZH professor of anthropology studying behavior of marmosets. Like Campbell’s monkeys, marmosets are small. Fully grown, they weigh between 250 and 400 grams and grow to a length of up to 30 centimeters (excluding the tail). It goes without saying that their brains are much smaller and far less powerful than the brains of humans or great apes. And yet, marmosets are keen communicators, and their babies babble like human babies do. Why is that? According to Judith Burkart, it has to do with their prosocial behavior. That is, the small monkeys in the Brazilian rainforest not only look out for themselves, but also for other members of their group.

Verankert ist dieses Verhalten in der gemeinsamen Aufzucht der Jungen. Anders als bei den Menschenaffen, bei denen die Mütter ihre Jungen alleine aufziehen, tun das die Krallenaffen gemeinsam, indem sich Pflegemütter und -väter abwechslungsweise um den Nachwuchs kümmern. Diese gemeinsame Brutpflege findet sich auch bei sozialen Insekten wie Ameisen oder Bienen, bei etwa neun Prozent der Vögel und rund drei Prozent der Säugetiere. Doch eine solche Form der Jungenaufzucht im Kollektiv wie bei den Krallenaffen kennen unter den Primaten nur noch wir Menschen.

This behavior is rooted in their cooperative breeding. Unlike in great apes, where mothers raise their young on their own, marmosets take turns caring for their offspring together with other male and female marmosets. This cooperative form of care has also been observed in social insects such as ants or bees, in about 9% of bird species and in around 3% of mammals. But the only other primates that care for their offspring collectively are humans.

This remarkable shared trait has moved the small monkeys into the spotlight of anthropologists. Their prosocial behavior is a key element in explaining why humans and great apes developed differently in terms of their language and cognitive skills. As mentioned, despite their small brains, marmosets are avid and complex communicators. Their repertoire of sounds is comparable to that of chimpanzees. They combine a wide variety of sounds and are masters at sharing information. Moreover, they also start calling back and forth as soon as they’re separated, and their turn-taking has the same structure we know from communication in humans.

Don’t you forget about me
Cooperative breeding requires a great deal of interaction. Among the various caregivers, for example, who have to coordinate duties such as care-taking or foraging for food. Or between adult marmosets and the young. The latter have to make sure that they receive enough food and attention, and crucially, that they’re not abandoned or forgotten about somewhere in the jungle. That’s why the young monkeys make noises, constantly. Similar to the babbling of human babies, they use elements of adult sounds that they string together at random. While this is risky and may attract predators, in evolutionary terms it must have been the better option than keeping quiet and being overlooked or forgotten.

Like human children, young marmosets endear themselves to their caretakers to make sure that they are looked after, explains Judith Burkart: “After all, interacting with an infant is more fun if we get a reaction.” We can’t help but feel a bit disappointed when we smile at a baby and it doesn’t smile back at us. Human and marmoset babies thus face the same evolutionary biological pressures: “They have to endear themselves to the adults and get their attention by interacting with them. And the better they are at doing so, the better their chances of survival,” says Burkart. This, in turn, is likely to feed back to the brain and result in the creation of new neural networks that promote communicative behavior.

With great apes, however, it’s a different matter. The young literally cling to their mothers, who care for them exclusively. And the mothers don’t depend on getting help from others to raise their young. This has some drawbacks, of course – if something happens to the mother, their young will also perish. And great ape mothers only give birth every few years, when their previous offspring are old enough to look after themselves. In marmosets, however, it’s the same as in humans, with females able to get pregnant again soon after giving birth.

Cooperative breeding in humans is now considered one of the key differences between humans and great apes. It allowed hominids to develop their bigger brains and thus break through the “gray ceiling”. Of course, larger brains take longer until they’re fully developed. This means that it also takes longer for offspring to reach maturity. If the mother cares for their baby alone and has to wait for it to become independent, the number of young she can have in her life is somewhat limited. This slows down the reproductive rate and may in extreme cases result in a species going extinct because it is unable to produce enough offspring to ensure the survival of the species.

No cooperation, no survival
Humans solved this problem by having their entire family or clan share caretaking duties. This prosocial behavior and the associated step-up in communication is also believed to have triggered the evolutionary leap in our language. Great apes communicate less among each other. “Because they don’t have to,” says Klaus Zuberbühler. “Great apes live in paradise.” In the jungle, they are literally surrounded by food and their survival does not depend on others. In contrast – as described in the Bible – humans left this paradise some time ago and ventured out into the African savanna, where living conditions are much harsher. “Adapting to life in the savanna led to massive changes, not least in terms of behavior,” Zuberbühler explains, “without cooperation, you won’t survive there.” This change in environment may not only have brought about cooperative child rearing, but also increased cooperation and communication when it came to finding food or fending off enemies.

Which brings us back to our initial question – what distinguishes us from our chimp relatives? One obvious answer might be the ability to control our impulses, which is likely also a consequence of our sizable brains. Add to that our prosocial behavior anchored in our collective child rearing, and this enabled hominids to develop larger brains and a complex language that can be used to coordinate their actions and pass on knowledge. The origins of human language evolution thus boil down to the following formula: a big brain plus prosocial behavior make for complex communication.

Judith Burkart believes that in the past six million years our brains and our language ability evolved together. Since our direct ancestors are extinct, we will likely never know for sure whether this is true. At best, we can measure the size of their brains using fossilized remains, but we can’t speak with them to find out about their use of language. We thus have no choice but to take a close look and listen carefully when monkeys, meerkats or birds communicate.

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